The present invention relates to a process for generating synchronizing signals used in an apparatus for optically transcribing data on a data carrier, during the write and/or read phases and in particular digital data recorded on a disk. It also relates to an optical device for performing this process.
Recording methods are well known to the Expert and fall outside the scope of the present invention. Generally the information is recorded in the form of microreliefs along tracks having a spiral or concentric circular configuration, the latter being easier to produce in the case of recording digital data. It in particular facilitates random access to recorded data, as well as a recording divided up into blocks or sectors.
No matter what the recording method, during reading it is necessary to have signals making it possible to synchronize said reading and numerous methods are also known for this purpose.
According to a first method in multitrack systems, with each track reserved for the recording of useful data is associated at least one other track along which are recorded miscellaneous data and in particular clock signals permitting the synchronization of the reading of the useful data. According to a variant, the tracks used for synchronization purposes are preetched and have optically detectable, regularly spaced disturbances. During reading, according to a first variant (multibeam system), a first beam is focused on the useful data track used for reading the data and a second, separate beam, but which is constantly mechanically coupled to the first, is used for reading the synchronization data. In a second variant (monobeam system), a single beam reads both sets of data. In this case, it is necessary that the two data types can be easily discriminated. For example, the frequency spectra associated with said sets of data can differ. In this variant, the preetched synchronization data can also be used during the writing phase.
In order to increase the recording density, it has also been proposed to use a single track, in which case the synchronization clock signals can be derived from the actual data reading.
In order to make synchronization easier, it is conventional practice to use autosynchronizing codes or a maximum of transitions, no matter what the content of the source information to be recorded. Thus, the microreliefs have two reference levels, respectively associated with logic values "0" and "1". The synchronization signals are derived from the detection of transitions from one given level to another and are used for the frequency and phase control of an oscillator, which is conventionally a voltage-controlled oscillator or VCO, equipped with a phase lock loop or PLL. In addition, special bursts of recorded pulses are used for initializing the control.
However, this type of coding does not permit a maximum recording density. For increasing this density, it is known to use non-autosynchronizing codes, e.g. the NRZ code (non-return to zero). The feature of this type of code is that there is no transition from one data bit to the other if the said two bits remain at the same logic value. It is then more difficult to derive the signals necessary for synchronization from the reading of the thus coded data.